The present invention relates to polymer films and methods for preparing polymer films. Specifically, the present invention relates to bioriented polyethylene films having high water vapor transmission rates (WVTR) and methods for preparing the same.
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages, including melt film formation, quenching, and windup. For a general description of these and other processes associated with film-making, see K R Osborn and W A Jenkins, Plastic Films: Technology and Packaging Applications Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
An optional part of the film-making process is a procedure known as xe2x80x9corientation.xe2x80x9d The xe2x80x9corientationxe2x80x9d of a polymer is a reference to its molecular organization, i.e. the orientation of molecules relative to each other. Similarly, the process of xe2x80x9corientationxe2x80x9d is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties).
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization, and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch, and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e. measurably higher stiffness and strength. Further, as a general rule, higher orientation correlates with lower WVTR values for films.
Previously, high WVTR values have been difficult to achieve with polyolefin films. Typically, film production methods aim to lower WVTR values for polyolefin films. As such, polyolefin films inherently have low WVTR values compared to traditional wrapping materials such as cellulose films or paper.
Accordingly, it is one of the purposes of this invention, among others, to produce bioriented polyethylene films having high WVTR values, by providing an economical and relatively uncomplicated method of making polyethylene films that imparts superior characteristics to the films, without requirement for chemically reactive additives, such as cross-linking agents, and without requirement for supplemental processing steps, such as irradiation of the film.
A multi-layer, bioriented film is stretched in the machine direction and in the transverse direction. This film may have high WVTR and comprises
(a) a base layer comprising polyethylene and a cavitating agent, said base layer having a first and second side; and
(b) skin layers on said first and second sides of said base layer, wherein at least one of said skin layers comprises (i) a hydrocarbon resin and (ii) a copolymer of ethylene and at least one monomer having at least three carbon atoms.
This film may be prepared by a method comprising the steps of:
(i) coextruding layers having the composition of said layers (a) and (b);
(ii) casting said coextruded layers of step (i) over a casting roll;
(iii) stretching said cast film of step (ii) in the machine direction; and
(iv) further stretching said stretched film of step (iii) in the transverse direction,
wherein said skin layer (b) is on the casting roll side of the film.
A package comprising this film may have a flattened tube shape. This flattened tube may be thought of as having a structure corresponding to that formed by bending the present film into a cylinder and adhering the contacted edges of the film to one another. The open ends of the cylinder or tube are then pressed together to flatten the tube.
The above-mentioned package may comprise a film with unidirectional tear properties in the machine direction. This package may have a lengthwise seal running in a direction perpendicular to two end seals. The machine direction of the film is parallel to the end seals and perpendicular to the lengthwise seal. The sealed package may contain multiple pieces of a food product, such as candy. This package is adapted for easy opening when held in a substantially vertical fashion, where one end seal is held above the level of the food product. The package is adapted to tear in a straight line along the machine direction at a level above the food pieces and at a level below the top seal to provide an opening above the level of the food pieces, such that these pieces do not fall out or spill when the package is opened.